Partial fix for #2095 (#2096)

* use immutable link in the initialstep for HMC

* bump patch version

* added test

* Update hmc.jl

---------

Co-authored-by: Hong Ge <[email protected]>

Author: torfjelde

src/mcmc/hmc.jl

@@ -131,13 +131,13 @@ function DynamicPPL.initialstep(
     rng::AbstractRNG,
     model::AbstractModel,
     spl::Sampler{<:Hamiltonian},
-    vi::AbstractVarInfo;
+    vi_original::AbstractVarInfo;
     initial_params=nothing,
     nadapts=0,
     kwargs...
 )
     # Transform the samples to unconstrained space and compute the joint log probability.
-    vi = link!!(vi, spl, model)
+    vi = DynamicPPL.link(vi_original, spl, model)
 
     # Extract parameters.
     theta = vi[spl]

test/mcmc/hmc.jl

@@ -246,4 +246,15 @@
             sample(demo_warn_initial_params(), NUTS(; adtype=adbackend), 5)
         end
     end
+
+    @turing_testset "(partially) issue: #2095" begin
+        @model function vector_of_dirichlet(::Type{TV}=Vector{Float64}) where {TV}
+            xs = Vector{TV}(undef, 2)
+            xs[1] ~ Dirichlet(ones(5))
+            xs[2] ~ Dirichlet(ones(5))
+        end
+        model = vector_of_dirichlet()
+        chain = sample(model, NUTS(), 1000)
+        @test mean(Array(chain)) ≈ 0.2
+    end
 end